Poly(ethylene terephthalate) may be derived from a process well known in the art comprising direct esterification of ethylene glycol and terephthalic acid. It may also be derived from a process comprising carrying out an ester interchange between ethylene glycol and dimethyl terephthalate to form bis-2-hydroxy ethyl terephthalate which is polycondensed to poly(ethylene terephthalate) under reduced pressure and at elevated temperatures.
Problems have also been encountered in the manufacture of poly(ethylene terephthalate) by the ester interchange reaction and by direct esterification reaction. However, this material is slow with respect to ester interchange and is many times impractical with respect to commercial operations. Therefore, it is typical to employ the use of a catalyst during these reactions.
Typical catalyst or catalyst systems for polyester condensation are well-known in the art. For example, catalysts disclosed in U.S. Pat. Nos. 4,025,492; 4,136,089; 4,176,224; 4,238,593; and 4,208,527, incorporated herein by reference, are deemed suitable in this regard. Further, R. E. Wilfong, Journal of Polymer Science, 54 385 (1961) sets forth typical catalysts which are useful in polyester condensation reactions.
However, many of these catalysts do not produce finished polyester which has the desired clarity or color.
U.S. Pat. No. 3,794,592 discloses a method of making polyesters suitable for use in making polyurethanes. More particularly, it discloses a method of making polyesters which have a reduced or controlled rate of reaction with isocyanates. Titanium may be used as a possible catalyst in the esterification and transesterification reactions. Phosphoric acid may be added after the esterification or condensation reaction has been completed. The molecular weight of these products are from about 500 to 10,000 and would not be useful in molding applications.
U.S. Pat. No. 3,907,754 discloses a catalyst system for the production of poly(ethylene terephthalate) which comprises, in combination, salts of manganese and cobalt with acetyl triisopropyl titanate and a phosphate ester.
U.S. Pat. No. 4,260,735 discloses the use of an alkanolamine titanium chelate catalyst to esterify and polycondense aromatic polycarboxylic acids or anhydrides, such as terephthalic acid, with a glycol, such as ethylene glycol, at low glycol to acid reactant ratios. The objective of this invention is to make possible commercial production of polyesters with low ether contents, without the addition of any inhibitor to suppress formation of DEG.
U.S. Pat. No. 4,150,215 discloses a process for preparing high molecular weight polyester resins by reacting ethylene glycol with terephthalic acid in the absence of a catalyst and polymerizing the product under solid state polymerization condition.
U.S. Pat. No. 3,962,189 discloses a catalyst-inhibitor system for the polymerization of poly(ethylene terephthalate) comprising a combination of organic or inorganic salts of manganese and cobalt, titanium alkoxides, organic salts of alkali metals, or alkaline earth metals, and a phosphate ester.
U.S. Pat. No. 4,356,299 discloses a poly(ethylene terephthalate) polycondensation catalyst system comprising a catalyst metal in the form of an alkyl titanate and an antimony compound.
Previously, the catalyst systems useful for making poly(ethylene terephthalate) of the prior art employed titanium-based catalysts, phosphorus-based compounds, usually in combination with manganese and/or antimony.
Therefore, there has been a need in the art to have a catalyst system which speeds up the reaction so that it is useful for economical purposes and for making a range of molecular weights useful for molding purposes but which also results in a condensation polymer having good clarity and color. Furthermore, environmental concerns demand that the level of catalyst metals be decreased or minimized which is achieved by using the process or this invention.
Heretofore, a catalyst system containing low amounts of one or more alkyl titanates in combination with a phosphorus-based compound in order to make poly(ethylene terephthalate) or modified poly(ethylene terephthalate) of sufficient molecular weight to be useful in molding applications has been unknown.